Method of analyzing mixed organic acid esters of cellulose



Patented Feb.- 9, 1937 UNITED- STATES PATENT OFFICE METHOD OF ANALYZING lflXED ORGANIC ACID ESTERS OF CELLULOSE No Drawing.

original application- March 1933, Serial No. 659,698.; Divided and this application November 24, 1934, Serial No.

The present invention relates to the determination of the contents of acetic acid, and of fatty acids of 3-4 carbon atoms in an aqueous solution by determining the percentage partition coef- 8 Claims. (gizpzso) phoric, in an amount which will exactly combine with ,the alkali metal content of the solution.

The Whole -is then subjected to distillation to separate off the organic acids preferably so that ficient of the solution in a water-insoluble liquid the normality of the distillate approaches 0.1 al

fatty acid ester of a lower aliphatic monohydroxy :Qtliough any concentration of distillate is satisfacalcohol. It also involves the determination of the content of acetyl and fatty acid groups of 3-4 carbon atoms in a cellulose ester. This application is a division of our application Serial Number 659,698, filed March 6, 1933. I

In view of the growing importance of the mixed .organic acid esters of cellulose such as cellulose acetate propionate and cellulose acetate butyrate a method of analysis whereby the percentages of the various acyl groups may be accurately determined is highly desirable. Various methods have been proposed for analyzing mixtures of fatty acids; however there is usually at least one disadjvantage met with in their practical use. For instance, some methods are suitable, when each of the acids constitutes a substantial percentage of the mixture; however when one of those acids is only about 5% or less the results are not sufficiently accurate, especially where butyric acid is present in small amount.

One object of our invention is to provide a method for analyzing for acetyl, propionyl and butyryl particularly when two or all of these groups are combined with cellulose. Another object of our invention is to provide a method for analyzing a mixture containing acetic, propionic and butyric acids in which the butyric acid may be present in small amount without abridging the accuracy thereof. Other objects will appear herein. k

The present invention involves the partition method for the analysis of acid mixtures by means of an extractant selected from the water-insoluble liquid fatty acid esters of the lower aliphatic monohydroxy alcohols, such as n-propyl acetate, n-butyl acetate, ethyl n-butyrate, etc. It also involves the hydrolysis of the acyl groups from the I cellulose ester containing them preliminary to the analysis proper.

Our method of analysis of, for instance a mixed organicacid ester of cellulose, is carried out by first treating the ester with a hydrolyzing base such as aqueous alkali until the acyl groups therein have all been hydrolyzed of! from the ester. The solution whichcomprises the alkali metal salts' of the organic acids and any alkali over that necessary to neutralize these groups is then treated with dilute high boiling mineral acid such as phos- -the indicator.

tory for carrying out the analysis with accuracy. By determining the amount of standard alkali necessary to neutralize a given amount of the distillate and by determining the percentage: partition between water and the extractant the perlated as described in the specific example.

Obviously this method of analysis'may be adapted to various modifications by-those skilled in the art, such as changing concentrations and still be within the scope of the present invention.

The following example illustrates the analysis of a cellulose ester containing acetyl, propionyl and butyryl groups by the present invention. Obviously if an ester containing-only acetyl and propionyl or acetyl and butyryl groups is to be 'centages of the various acyl groups may be calcuanalyzed the calculations need not be as involved as when there are three different acyl groups present, however, often the cellulose acetate propionate produced commercially has a small butyryl content and c mmercial cellulose acetate butyrate may contai some propionyl so that the method described in the example will ordinarily be used. v

s A 2 gm. sample of the cellulose ester to be analyzed is placed in a 500 c. c. round bottom short neck flask and treated with c. c. of 0.5 N alkali (aqueous sodium or potassium hydroxide).

E The flask and contents are maintained at 40 C. for 48 hrs. The solution is then acidified with molar phosphoric acid, the volume of phosphoric acid necessary havingbeen previously determined by titration of a sample of the .5 N alkali with the phosphoric acid. Methyl orange is employed as (The use of methyl red is not advised for the most accurate work because its color changes at a higher pH value than methyl orange.)

The flask is then fltted with a two hole rubber stopper equippedwith a Kieldahl trap (for spray) and a capillary tube for the vacuum distillation of the, organic acids. An ordinary straight inner tube condenser is connected to the Kjeld'ahl trap and fa-500 c. c. Engler distillation flask is used for the receiver. The end of the condenser should extend down into the receiver flask which is packed with ice and salt (or cooled below 0 C. by any other appropriate means). The distillation flask is immersed in a 5" water bath whose temperature is about 90 0.

Operating at this-temperature with a pressure in the system of about m. m. the flask becomes dry after about 2 hours. The distillation must be continued after the flask is apparently dry in order to remove the saturated vapor and liquid which collects in the trap.

When the flask and bulb are dry, the trap and flask are rinsed with about 10 c. c. of water and the distillation is again carried to dryness after which this operation is repeated. The condenser 65 tate is finally rinsed'into the receiver and the distillation will approximate five hours.

Twenty-five c. c. of the distillate are titrated with standard alkali. The volume of alkali required is designated M. Thirty c. c. 01 the distillate are then extracted with 15 'c. c. of n-propyl acetate, the aqueous layer being removed and c. c. of it titrated with the standard alkali. The volume of alkali required for this titration is designated as M1. The extraction is again carried out using c. c. of the distillate with c. c.

of n-propyl acetate. The volume of alkali necessary to titrate 25 c. c. of the aqueous'phase in this case is designated as Mr. These values are substituted in the equations in which K1 and K: are termed the percentage partition coefilcients. They represent the.percentage of the original acid which remains in the aqueous phase after the extraction in each case. If the value is desired in a decimal fraction instead of in percentage the multiplication by 100 will or course be omitted. From these values the acetyl, propionyl and butyryl contents of the ester can be calculated as follows:

In the following equations let k1=percentage partition coefllcient of 30 c. c. of .1 N acetic acid with 15 c. c. of propyl acetate.

70:;percentage partition coeiilcient of 30 c. c. 01!..1- N propionic acid with 15 c. c. of propyl acetate.

ka=percentagepartition coefllcient o! 30 c. c. of .1 N butyric acid with 15 c. c. of propyl acetate.

k4==percentage partition coeflicient of 30 c. c.

of .1 N acetic acid with 60 c. c. of propyl acetate. ks=percentage partition coefllcient o! 30 c. c. of .1 N propionic acid with 60 c. c. of propyl aceks=percentage partition ooefllcient of 30 c'. c. of .1 N butyric acid with 60 c. c. of propyl acetate.

These values are determined in the laboratory by extraction of .1 normal solutions of pure acids in the same manner as described above for an unknown solution. These values upon determination in the laboratory were found to be as follows:

Acetic acid Propionic acid .Butyric acid In order to evaluate three unknown quantities one must have three simultaneous algebraic equations involving them. In the case of a ternary acid mixture obviously the sum of the mol. percentages'of the acids present will be If A, P and B are allowed to represent respectively is obtained and 6 as determined, the equation -simplifies to .P=9.95 (xi -81.6) 12.23 (Ks-53.4)

In a similar manner, B is round to be expressed by the equation,

As A=100P- -B the mol. per cent of acetic acid may be readily calculated. Obviously this method is also applicable to the analysis 01 cellulose acetate propionate and cellulose acetate butyrate. v

In the case where the third acyl is substantially entirely absent there need be carried out only one extraction for instance with 15 c. c. (or with 60 c. c. as desired) of the extractant and the value for only one percentage partition coefiicient which is designated by K1 or by K:

is'necessary. As there are only. two unknowns only two simultaneous algebraic equations are necessary and these may be for instance:

Akl+Pk 2 =K1 where propionyl and acetyl-only are involved and it is desired to use 15 c. c.,of the extractant in the extractions.

Solving these equations for P 1o'ok,-K,. p l .z is obtained If it is desired to use 60 c. c. of the extractant the equations will be:

- Alt4+Plts=Kz from which 1ook,K, I P=-- k .18 obtained Where acetyl and butyryl only are involved and using 15 c. c. of extractant the equations may be:

- A+B=1OQ Alci+Blcs=Ki from which g gi z is obtained By substituting the values of It; and. leg, as determined, in the formula g g .1o0k1K,

I kf-k, the equation simplifies to:

M= 32.33 c. c. 37.20 c. o M 23.78 c. c. 27.27 c. c Mg=100 M 14.53 c. c. 16.62 c. c

i K1 1-o0 M 73.5 7 3.3

After substitution of K1 and K2 in the appropriate equations given, the molar percentages of the three acids in the distillate are found to be as follows:

Percent Percent A: 73. 2 72. 6 P= 23. 4 23. 8 B= 3. 4 3. 6 r

The apparent acetyl value (all the acyl cal-' culated in terms of acetyl) is 42.2%. Thus the percentage ofeach of the three acyl groups in th e ester will be found to be:

Acetyl- 30. 9 30. 5 Propionyl l3. 1 l3. 3 Butyry 1 2. 2 2. 5

As was pointed out above the present invention also includes the analysis of a mixture of two or all of the fatty acids of 2-4 carbonatoms, and is accurate even when butyric acid is present in only small amounts. The following data illustrate the accuracy of this analysis with various mixtures of the three acids in varying proportionsz" Acetic Propionic Butyric Percent Percent Percent 51. 1 2a 6 24. 3 22-; i? Mixture 2 {gakern 2513 23: 2 g oun 21. 5 t ae2 i8"; 011D W #4 {Taken 10. 2 2s s 65. 2

As pointed out above other liquid fatty acid esters of the lower monohydroxy alcohols which are substantially insoluble in water such as n-butyl acetate, ethyl n-butyrate, ethyl propionate, isobutyl acetate, n-propyl butyrate and the like are suitable for use as' extractants in the analysis of cellulose esters in accordance with the present invention. As n-butyl acetate has a comparatively wide range from the value of K1 for acetic acid tothe value of K: for butyric acid it is particularly desirable for use in accordance with the present invention.

We claim as our invention:

1. A method of analysis of a mixture of fatty acids of 2-4 carbon atoms which comprises titrating a given quantity of the mixture with standard alkali, extracting a measured amount of the mixture in aqueous condition with a predetermined amount oi an extractant comprising a liquid fatty acid ester of a lower aliphatic 'monohydroxy alcohol which is substantially insoluble in water, removing the aqueous layer therefrom, titratlng an amount of that aqueous layer, corresponding to the amount of mixture which 'ts titrated, with standard alkali, extracting ano ther like amount of the mixture in aqueous condition with an excess of the same type of extractant, removing the aqueous layer and titrating an amount, corresponding to that titrated above, with standard alkali, determining the percentage partition coemcient designated as K1 and K2 in accordance with the formulas:

where M represents the amount of alkali used in the titration of the mixture before extraction, M1 representsthe amount of alkali used in the titration. of the aqueous layer when a small amount of extractant was used and M2 represents the amount of alkali used in titration of the aqueous layer when a large amount of extractant was used, and then mathematically determining the contents of the various acids in the mixture from these percentage partition coefiicients and the percentage partition coemcients of the inemployed.

2. A method of analysis of a cellulose ester containing a plurality of fatty acid groups of 2-4 carbon atoms which comprises completely hydrolyzing .the ester with aqueous alkali, treating with a high boiling mineral acid in an amount dividual acids in the two amounts of extractant chemically equivalent to the alkali metal content, distilling the resulting mass to obtain all of the lower fatty acid in the form of a distillate, titratinga given amount of the distillate with standard alkali, extracting a measured amount of the distillate with a predetermined amount of an extractant comprising a liquid fatty acid ester of a lower aliphatic monohydroxy alcohol which is substantially insoluble in water, removing the aqueous layer therefrom and titrating an amount, corresponding with the amount of distillate previously titrated, with standard alkali, extracting another like amount of distillate with an excess of a like extractant, removing the aqueous layer and titrating'with standard alkali as before, determining the percentage partition coefiicients which may be designated K1 and K2 in accordance with the formulas:

K IOIM 1ooM,

in the titration of the distillate before extraction M1 represents the amount of alkali used in.the

titration of a given amount of aqueous layer when a'sn'iall amount of extractant was used and then mathematically determining the contents of the various acyls from these percentage partition coefllcients and the percentage partition coefllcients of the individual acids of the two amounts of extractant employed.

3. A method of analysis of a mlxture'of acetic acid with only one fatty acid of 3-4 carbon atoms which comprises titrating a given amount of the mixture with standard alkali, extracting a measured amount of the mixture with a liquid fatty where 101 is the percentage partition coefficient where M represents the amount of alkali usedin the titration of the distillate before extraction, and M1 represents the amount of alkali used in the titration of the aqueous layer formed when.

the extractant was employed on the mixture, then determining the propionyl or butyryl in accordance with the following formula:

where 701 is the percentage partition coefllcient of acetic acid with the amount of extractant employed in the extraction and a is the percentage partition coefficient of the fatty acid of 3-4 carbon atoms with the amount of extractant employed in 'the extraction.

4. A method of. analysis of a cellulose ester containing only acetyl groups and groups of one fatty acid of 3-4 carbon atoms which comprises completely hydrolyzing the ester with aqueous alkali, treating with a high boiling mineral acid in an amount chemically equivalent to the alkali metal content, then distilling to obtain all the lower fatty acids in the distillate, titrating a givenamount of the distillate with a liquid fatty acid ester of a lower aliphatic monohydroxy alcohol which is substantially insoluble in water, removing the aqueous layer therefrom and titrating an amount of this aqueous layer, corresponding to that of the distillate titrated, with standard alkali, determining the percentage partition coefiicient which may be designated K1 in accordance with the formula:

100M TF where M represents the amount of alkali used in the titration of the distillate before extraction and M1 represents the amount of alkali used in the titration of the aqueous layer when a small amount of extractant is used, then determining the propionyl or butyryl in accordance with the following formula:

of acetic acid with the amount ofextractant employed in-the extraction and a is the percentage partition coefficient of the fatty acid of 3-4-carbon atoms with the amount of extractant employed in the extraction.

5. In the method of determining the percentage partition coeflicient of a mixture of fatty acids of- 2-4 carbon atoms to be used in the determination of the acyl content of that mixture the steps which comprise extracting the mixture in aqueous condition-in a predetermined amount of an extractant comprising a liquid fatty acid ester of a lower aliphatic monohydroxy alcohol which is substantiallyinsoluble in water and then titrating a given amount of the aqueous layer formed by that extraction with standard andtitrating a given amount of the aqueous layer with standard alkali.

7. In a method of determining a percentage partition coefficient of a mixture of fatty acids of .2-4 carbon atoms for use in determining the relative acyl contents of thatmixture, the steps which comprise extracting a given amount of the mixture with the mixture in aqueous condition with a predetermined amount of normal butyl acetate, removing the aqueous layer therefrom and titrating a given amount of the aqueous layer with standard alkali.

8. A method of analysis of a cellulose ester containing a plurality of fattty acid groups of 2-4 carbon atoms which comprises completely hydrolyzing the ester with aqueous alkali, adding thereto a high boiling acid in an amount chemically equivalent to the alkali metal content, distilling the mass to obtain all the lower fatty acidtherefrom as a distillate, titrating a given amount of that distillate, extracting a measured amount of the distillate with normal propyl acetate, removing the aqueous layer and titratin an amount of that layer, corresponding to th amount of distillate titrated, with standard alkali, extracting another like measured amount of distillate with an 'excess of normal propyl acetate, removing the aqueous layer and titrating an amount of that layer, corresponding to the amount of the distillate titrated, with standard alkali, determining the percentage partition coeflicients which may be designated K1 and K: in accordance with the formulas:

where M represents the amount of alkali used in the titration of distillate before extraction, M1 represents the amount of alkali used in the titration of the aqueous layer when a small amount 1 of extractant was used and M: represents the amount of alkali used in the titration of the aqueous. layer when a large amount of extractant was used and then, calculating the propionyl content P in accordance with the following formula:

in which In equals the percentage partition coefficient of an amount of .l N acetic acid corresponding to 'the amount of aqueous layer extracted with the smaller amount of propyl acetate employed in the extraction, In: is the percentage partition coefllclent under like circumstances of propionic acid, ks is the percentage partition coefficient under like conditions of butyric acid, k4 is the percentage partition coeflicient of the designated amount of the 0.1 N acetic acid with the excess quantity of propyl acetate used in extraction, k5 and kc are the percentage partition coefficients under like conditions of propionic and butyric acid respectively, or the butyryl'content B in accordance with an equivalent formula.

CARL J. MALM. GALE F. NADEAU. 

